Process for the electrochemical treatment of concrete

ABSTRACT

A process for the electrochemical realkalization of reinforced concrete comprises passing a direct electric current between an anode associated with a layer of alkaline electrolyte applied to an external surface of the concrete and a cathode which is located internally in the concrete. The process causes the internal pH of the concrete to increase and a surface layer of the concrete to be impregnated with the electrolyte solution and which comprises a solution of potassium carbonate of concentration at least 0.3 Molar. The process may be applied to concrete a zone of which has a pH of less than 10.0 and the process continued until the pH reaches a level of at least 10.5, preferably at least 11.0.

This application is a 371 of PCT/GB98/01967 filed Jul. 3, 1998.

FIELD OF THE INVENTION

This invention relates to a process for the electrochemical treatment ofreinforced concrete.

BACKGROUND OF THE INVENTION

In reinforced concrete the steel reinforcement is normally protectedagainst corrosion by the alkaline environment in the concrete mass.Gradually however the alkalinity is reduced by the action of carbondioxide and other gases in the atmosphere such as oxides of sulphur. Theterm given to this process is carbonation and concrete which has beensubjected to the action of atmospheric gases is referred to ascarbonated.

The effect is a gradual decrease in the pH and, if the process isallowed to continue, the pH will continue to fall and when it reaches avalue of about 9.5 the steel is no longer protected against corrosion.Corrosion of the reinforcement may then take place leading to weakeningof the reinforcement and spalling of the concrete.

A process for increasing the alkalinity of concrete which has beencarbonated has been previously described, for example in European PatentNo. 264,421 and U.S. Pat. No. 4,865,702. In this process an electriccurrent is passed between an external electrode which is in contact withan alkaline electrolyte applied to the external surface of the concreteand the internal reinforcement of the concrete as cathode.

During the process there are two effects: the alkaline electrolyte movesinto the concrete and the alkalinity is increased in the region of thecathode. The benefit of this is that the steel is repassivated and alayer of electrolyte which has permeated into the concrete maintains acover zone over and around the steel of sufficiently high pH to keep thesteel passive. This process has become known as realkalization.

As the alkaline electrolyte for this process there is proposed inEuropean Patent No. 264,421 an aqueous solution of calcium, sodiumand/or potassium salts either in a liquid or absorbed in a porous mediumsuch as rock wool, cellulose, sawdust, sand, clay and the like or theelectrolyte can be strongly retarded concrete, mortar, cement grout orlime paste. GB Patent Aplication No. 2,271,123A discloses a process inwhich sodium carbonate or sodium borate is used in the aqueouselectrolyte. However in the operation of the process on a commercialscale the alkaline electrolyte that has been used is an aqueous solutionof sodium carbonate.

PROBLEM TO BE SOLVED BY THE INVENTION

Although the results of the process using sodium carbonate solution asthe alkaline electrolyte have been generally very successful, thesurface of the concrete after the treatment, frequently showsefflorescence. This is a heavy crystalline salt deposit which isunsightly and is unsuitable for the application of a decorative coating.Consequently it is necessary to thoroughly clean the concrete aftertreatment to remove the efflorescence.

A solution to the above described problem has now been invented in whicha solution of potassium carbonate is employed as the electrolyte.

SUMMARY OF THE INVENTION

According to the present invention a process for the electro-chemicalrealkalization of concrete comprises passing a direct electric currentbetween an anode in contact with a layer of aqueous electrolyte appliedto an external surface of the concrete and a cathode which is locatedinternally in the concrete, to cause the internal pH of the concrete toincrease and surface layer of the concrete to be impregnated with theelectrolyte solution and wherein the electrolyte layer contains anaqueous solution of potassium carbonate at a concentration at least 0.3Molar.

ADVANTAGEOUS EFFECT OF THE INVENTION

The advantage of the present invention obtained by the use of an aqueoussolution of potassium carbonate as the alkaline electrolyte is thatefflorescence on the external surface of the concrete after treatment isreduced or eliminated. This advantage could not have been predicted fromthe prior art and is therefore unexpected.

The FIGURE is an illustration of an electrolytic cell for the treatmentof concrete.

DETAILED DESCRIPTION OF THE INVENTION

By external surface is meant a surface that is exposed to theatmosphere. The term electrolyte is intended to refer to the aqueoussolution of potassium carbonate.

The anode may be immersed in the electrolyte solution or in someembodiments of the process associated with an adherent coating whichcomprises an organic water retaining material which forms an adhesivemixture with water.

The concentration of the potassium carbonate solution is preferably atleast 0.5 Molar and solutions of concentration from 0.5 Molar up tosaturation concentration are particularly suitable.

The potassium carbonate can be generated in situ from a source ofpotassium ions and a source of carbonate ions. For example, potassiumhydroxide as the source of potassium ions and lithium carbonate as thesource of carbonate ions may be added to the water to make up theelectrolyte. Preferably however the electrolyte will be substantiallyfree of ions other than those derived from the potassium carbonate andwater although small quantities of other ions eg calcium, sodium andlithium (for example in the amounts that occur in commercially availableforms of potassium carbonate) are acceptable. It is therefore convenientto add potassium carbonate (as the compound) to the water to make up theelectrolyte. The potassium carbonate may be a general industrial gradematerial for example one containing at least 97% by weight of potassiumcarbonate on a dry basis.

It is preferred that the amount of sodium ions, if present, is less than5% by weight of the potassium carbonate based on the dry weight of thepotassium carbonate, because the inventors have found that sodiumcarbonate is the material which causes the efflorescence on the concretesurface after treatment.

Preferably the pH of the electrolyte at the commencement of the processis in the range 10.5 to 12.5, more preferably 10.9 to 12.0.

The electrolyte may be maintained in contact with the external surfaceof the concrete by an adherent coating which comprises an adhesivemixture of an organic water retaining material and water.

Alternatively the electrolyte is maintained in contact with the externalsurface of the concrete by means of a tank which holds the electrolyteand is removably secured to the concrete.

When an adherent coating is used the adhesive mixture may be applied byspraying and the mixing of the water with the composition containingpotassium carbonate and the water retaining material effected in thespraying process.

The water retaining material is conveniently a cellulosic fibre, forexample, as described in European Patent No. 398,117 or U.S. Pat. Nos.5,198,082; 5,228,959; and 5,407,543 and suitably the composition ofcellulosic fibre and potassium carbonate contains at least 10% by weightof potassium carbonate based on the dry weight of cellulosic fibre,preferably from 20 to 150% by weight of potassium carbonate based on thedry weight of cellulosic fibre.

The adhesive mixture conveniently contains more potassium carbonate thanis required to saturate the water present in the mixture. In this casethe coating will contain undissolved potassium carbonate which acts as areservoir to for replenishment of the electrolyte. In this casereplenishment may be effected by addition of water for example byspraying the coating at intervals. When the electrolyte solution iscontained in a tank, replenishment may be effected by addition of freshsolution to the tank.

During the process the electrolyte may be replenished. Conveniently thewater retaining material can hold at least 100% its own weight of waterand preferably at least 200%, more preferably at least 300% for example300 to 500%.

When the water retaining material is a cellulosic fibre, the fibre canbe premixed with the potassium carbonate, e.g. at a factory so that onthe site only water needs to be added to the fibre.

For use in the present invention the dry fibre is conveniently mixedwith the potassium carbonate (as a solid) in the process for thepreparation of the fibre e.g. the milling of the cellulose and suppliedto the job site as a mixture where it is mixed with water, for example,by supplying the cellulose fibre/potassium carbonate mixture and wateras two components to a suitable spray nozzle in which they are mixed andfrom which is emitted a spray containing a mixture of the two.

The cellulosic fibres may be recycled or reconstituted cellulose pulp.Conveniently the cellulose pulp is derived from newsprint or other wastepaper. Processes for the production of cellulose fibres are known in theart and are in commercial operation. Cellulose fibre is known as areplacement for asbestos fibres in a number of applications such aspanels, tile adhesive, refractory linings and especially fibre cementpanels.

In a typical process for the preparation of cellulose fibres the feed inthe form of waste newsprint in sheet form is passed to a shredder fromwhere the shredded paper is passed through the first of two hammer millssuch as a Jacobson mill. The mill has rotating hammers or blades whichtogether with air suction force the material through a perforated metalscreen. The material, which at this point is partly fibrised, is passedto a second hammer mill. At a point between the two hammer millschemicals such as fire retardants are added. In the preparation of fibrefor use in the present invention the potassium carbonate is added atthis point. The material is then passed through the second hammer millin which it is further fibrised. The product is then compressed andextruded into bags for storage. The fibres after leaving the secondhammer mill usually have a length of between 0.5 and 2.0 mm. Thefreeness of the fibres may be in the range 45 to 75o SR(Shopper-Riegler).

The cathode is conveniently connected to, or provided by, thereinforcement of the concrete.

The process is particularly suitable for use with concrete that has beencarbonated to a pH of 10.5 or less, such as 10.0 or less, especially 9.5or less, since at this pH the steel of the reinforcement is no longerprotected against corrosion.

The process of the present invention can be carried out as described inEuropean Patent No. 264,241 and U.S. Pat. No. 4,865,702.

For example the applied voltage between anode an cathode canconveniently be from 3 to 40 volts conveniently from 6 to 20 volts andis usually adjusted to provide a current density in the range from 0.15to 6 preferably from 0.5 to 2.5 amps per square meter of concretesurface.

The alkalinity can be monitored by measuring the pH e.g. by means of anindicator such as phenolphthalein sprayed onto freshly broken concreteand when the desired pH has been reached e.g. a pH of greater than 10.5,usually greater than about 11, the process can be stopped.

Since loss of alkalinity is caused by atmospheric gases such as carbondioxide, for a concrete that has weathered, the pH at or near thesurface will often be lower than that further into the concrete body andthe pH in the immediate vicinity of the reinforcement may still besufficiently high to provide passivation of the steel. It is within thescope of the present invention to realkalize such concrete.

Although the pH of the concrete may vary through its thickness any zonemay be chosen as the zone whose pH is measured to determine whether torealkalize the concrete. The zone whose pH is measured to determine theend of the process will usually be the same distance from the surface asthe first chosen zone.

The process may comprise measuring the pH of a chosen zone of theconcrete and when the pH is 10.0 or less, carrying out the process asdescribed above and, after a period of time measuring the pH again, andwhen the pH is 10.5 or more stopping the passage of the electriccurrent.

The anode which can comprise wires, cords, plates, foil or sheet metaland its associated electrolyte can be preformed and applied to theconcrete surface as an assembly.

The anode can be a consumable metal such as steel or an inert metal suchas titanium.

The electrolyte is conveniently maintained in contact with the concretesurface by means of a tank which holds the electrolyte and which isremovably secured to the concrete surface. The use of a holding tankenables the electrolyte to be maintained in contact with sloping,vertical and overhead surfaces and enables the process to be applied tothe underside of concrete structures such as soffits or ceilings and thelike.

Referring to the drawing a body of concrete 2 has an external surface 4,steel reinforcement 6 and a zone 8 adjacent to the surface 4 which hasbeen carbonated. Maintained in contact with the external surface 4 bymeans of a removable tank (not shown) is an electrolyte 10 which is a 1Molar aqueous solutuion of potassium carbonate. Immersed in theelectrolyte 10 is a steel anode 12. In operation of the process of thepresent invention a voltage is applied between the reinforcement 6 ascathode and the steel anode 12 to provide a direct current at a currentdensity of 1 amp per square meter of concrete surface.

Electrolyte 10 penetrates the surface 4 of the concrete and into theconcrete as indicated by the downward pointing arrows. A front ofadvancing electrolyte is indicated at 16. This is shown as a mixture ofpotassium carbonate and bicarbonate because potassium carbonate isconverted by carbon dioxide in the atmosphere to the mixture (notnecessarily in equimolar proportions). Under the influence of theelectric current the ions move in the directions indicated ie potassiumand sodium ions move towards the cathode and hydroxyl ions towards theanode.

The invention is illustrated by the following Example.

EXAMPLE 1

A reinforced concrete structure that had been carbonated by exposure tothe atmosphere over a period of years and whose pH in the vicinity ofthe steel reinforcement had fallen to about 9.5 was subjected to thefollowing treatment.

The concrete structure treated was a reinforced concrete soffit (thisprovides an overhead surface). For the treatment of this overhead area atank was used to hold the electrolyte so that the latter was maintainedin direct contact with the concrete surface. The tank of dimensions 1100mm×900 mm×10 mm was made of 4 mm perspex sheet with 40 mm×25 mmpolyethylene or neoprene seals at the edges and included a 20 mm×20 mmmild steel frame to provide rigidity and compress the seals. The tankwas secured to the concrete by means of bolts. The tank contained a oneMolar aqueous solution of potassium carbonate whose pH was 12.0.

A mixed metal oxide coated titanium mesh (210 grade) was held in thetank immersed in the electrolyte solution and connected to the positiveterminal of a source of direct current whilst the reinforcement wasconnected to the negative terminal. The voltage was adjusted to providea current density of one amp per square meter of concrete surface andwas applied for 4 days. The pH of the concrete in the zone between thereinforcement and the concrete surface was measured after treatmentusing a phenolphthalein indicator made up in water and ethanol which wasturned bright pink indicating a pH of about 11.

The tank containing the electrolyte solution was removed and theexternal surface of the concrete inspected after drying. Noefflorescence was observed.

The Example was repeated exactly as described above except that a 1Molar solution of sodium carbonate was employed. After removal of thetank and drying, the surface was inspected and efflorescence wasobserved, which it would be necessary to remove before applying adecorative coating.

Experimental work has found that potassium carbonate has furtheradvantages over sodium carbonate:

(i) potassium carbonate solution penetrates into the concrete fasterthan sodium carbonate under identical conditions and molarconcentrations. This means that the pH of the concrete layer adjacent tothe surface is increased more quickly.

(ii) potassium carbonate has far better solubility properties at lowtemperature. For example at 4 degrees Centigrade sodium carbonate at asaturated solution is below 1 Molar. Potassium carbonate however has asaturated solution of over 5 Molar at this temperature. This issignificant since one of the main applications of this invention is forthe treatment of the external surfaces of buildings and other concretestructures and means that potassium carbonate can be more reliably usedduring the winter.

What is claimed is:
 1. A process for the electro-chemical realkalizationof reinforced concrete comprising passing a direct electric currentbetween an anode in contact with a layer of aqueous alkaline electrolyteapplied to an external surface of the concrete and a cathode which islocated internally in the concrete, to cause the internal pH of theconcrete to increase and a surface layer of the concrete to beimpregnated with the electrolyte solution and wherein the aqueouselectrolyte solution comprises an aqueous solution of potassiumcarbonate of concentration at least 0.3 Molar.
 2. A process as claimedin claim 1 wherein the electrolyte is maintained in contact with theexternal surface of the concrete by an adherent coating which comprisesan adhesive mixture of an organic water retaining material and water. 3.A process as claimed in claim 2 wherein the adhesive mixture containsmore potassium carbonate than is required to saturate the water presentin order to provide a reservoir of potassium carbonate to replenish theelectrolyte.
 4. A process as claimed in claim 2 wherein the waterretaining material is a cellulosic fibre and the amount of potassiumcarbonate present is at least 10% based on the combined dry weight ofthe cellulosic fibre and potassium carbonate.
 5. A process as claimed inclaim 4 wherein the amount of potassium carbonate present is from 20 to150% by weight based on the combined dry weight of cellulosic fibre andpotassium carbonate.
 6. A process as claimed in claim 1 wherein theelectrolyte is maintained in contact with the external surface of theconcrete by means of a tank which holds the electrolyte and is removablysecured to the concrete.
 7. A process as claimed in claim 1 wherein theapplied voltage between anode an cathode is from 3 to 40 volts and thecurrent density is from 0.15 to 6 amps per square meter of concretesurface.
 8. A process as claimed in claim 7 wherein the applied voltageis from 5 to 20 volts and the current density from 0.5 to 2.5 amps persquare meter.
 9. A process as claimed in claim 1 wherein the process isapplied to concrete a zone of which has a pH is less than 10.0.
 10. Aprocess as claimed in claim 9 wherein the process is continued until thepH reaches a level of at least 10.5, preferably at least 11.0.